Fingertip pulse wave sensor

ABSTRACT

A fingertip pulse wave sensor includes a fingertip cushion supporting member (10) having a fingertip cushion supporting surface (11), and a nail supporting member (20) having a nail supporting surface (21) which is disposed in opposed relationship with the fingertip cushion supporting surface (11) at an interval smaller than the standard thickness of fingertips (1). Either of the fingertip cushion supporting member or the nail supporting member (20) is fixed, and the other of the fingertip cushion supporting member (10) and the nail supporting member is hinged. The hinged fingertip cushion supporting member (10) or nail supporting member is urged in the direction in which they are moved to each other by an elastic body (30) which is capable of applying a substantially fixed amount of pressure with respect to the variations in the pivot stroke caused by the variations in the thickness of the fingertips (1) resting on the fingertip cushion supporting member (10). The fingertip pulse wave sensor also includes a light-emitting element (15) which is disposed on either of the fingertip cushion supporting surface (10) and the nail supporting surface, and a light-receiving element (26) which is disposed on the other of the fingertip cushion supporting surface and the nail supporting surface.

This is a continuation of copending application Ser. No. 07,329,579filed on Mar. 28, 1989 now U.S. Pat. No. 4,971,062.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fingertip pulse wave sensor fordetecting changes in the volume of a blood vessel in a fingertip heldbetween a light-emitting element and a light-receiving element, in whichthe changes are caused by pulsation and detected as electrical signalsproduced in response to changes in the intensity of the lighttransmitted through the fingertip.

2. Description of the Related Art

A conventional fingertip pulse wave sensor of the above-described typeincludes a pad with either a light-receiving element or a light-emittingelement buried therein which makes contact with a fingernail, and a padwhich has the other of the light-receiving and light-emitting elementsburied therein and which makes contact with a fingertip cushion. Thesepads are disposed in opposed relationship with each other. Theconventional fingertip pulse wave detector also includes a plate springhaving a U-shaped form and is mounted on the rear surface of either ofthe pads so as to hold the fingertip between the two pads and underpressure.

With this arrangement, although the fingertip can be elastically held inplace in spite of the variations naturally occurring in the shape of thefingertips of different individuals or any irregularities in the topdead point of the plate spring employed, these variations orirregularities serve to cause variations in the amount of pressureapplied to the finger to a considerable extent. For example, in the caseof a spring designed to provide a force that will press a fingertip witha deflection of 3 mm, if a deflection of 6 mm is caused due todifferences in the shape of different individuals' fingertips orirregularities in the top dead point of the spring employed, it ispossible that the pressure applied to the fingertip will be completelydifferent. Excessive pressure exerted on a portion to be measured,increases the amount of blood in the venula of the fingertip, which isbeing returned to the associated vein. This has an effect on thewaveform of a pulse, which is produced during arterial pulse wavedetection at the fingertip.

This is undesirable in terms of the measurement precision in aplethysmograph which is designed to estimate the form of a waveform inits detected state. This greatly affects the results of measurementsobtained using an "acceleration type" fingertip pulse wave detectorwhich must be able to non-invasively monitor the state of a peripheralcircle by converting the waveform detected to an acceleration curve.This conversion is achieved by differentiating twice the detectedwaveform with respect to the time. This conversion requirement, ineffect, prevents such an "acceleration-type" fingertip pulse wavedetector from being employable in clinical applications.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide afingertip pulse wave sensor which is capable of detecting a fingertippulse wave in a consistent manner and with a high degree of accuracy inspite of differences naturally occurring in the shape of fingertipsamong different individuals.

In order to achieve this object, the present invention provides afingertip pulse wave sensor which is based on the confirmation thatapplication of a certain amount of pressure is inevitable for stablepulse wave detection.

The fingertip pulse have sensor of the present invention, comprises afingertip cushion supporting member having a fingertip cushionsupporting surface, and a nail supporting member having a nailsupporting surface which is disposed in opposed relationship with thefingertip cushion supporting surface at distance (i.e., interval)smaller than the standard thickness of fingertips. Either of thefingertip cushion supporting member and the nail supporting member arefixed while the other is hinged. The fingertip pulse have sensor furthercomprises an elastic body for urging the hinged fingertip cushionsupporting member or nail supporting member in the direction in whichthey are moved toward each other in such a manner that the pressureapplied to the hinged fingertip cushion supporting member or nailsupporting member becomes constant with respect to variations in thepivot stroke. Notably, these variations in the pivot stroke are causedby the variations in the thickness of the fingertips resting on thefingertip cushion supporting surface. This hinging feature of thepresent invention enables the adoption of various types of springstructure which are capable of maintaining the pressure applied at afixed value, unlike a plate spring which directly presses a fingertip.

Present invention allows a fingertip to be gripped between photoelectricelements under a substantially fixed and appropriate pressure which issufficient to keep the fingertip in place, and yet not to compress ablood vessel to the extent of affecting the measurement accuracy. Thisenables detection of a stable pulse waveform signal accuratelycorresponding to changes in the volume of a blood vessel.

In particular, application of the present invention to anacceleration-type pulse wave detector which requires that a stable pulsewaveform is detected with a high degree of accuracy, enables highlyreliable monitoring of the conditions of peripheral circulation. Thisfeature of the present invention, expectedly will open the way to use ofthe acceleration-type fingertip pulse wave detectors in clinicalapplications, including prevention and cure of degenerative diseasescaused by circulatory disorders.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the central portion of a fingertippulse wave sensor, showing a first embodiment of the present invention;

FIG. 2 is a section taken along the line 2--2 of FIG. 1;

FIG. 3 is a cross-sectional view of the central portion of a fingertippulse wave sensor, showing a second embodiment of the present invention;

FIG. 4 is a section taken along the line 4--4 of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show a first embodiment of a fingertip pulse wave sensoraccording to the present invention.

In this embodiment, the rear end portion of a fingertip cushionsupporting plate 10 which acts as a fingertip cushion supporting member,is hinged on shaft rods 7 protruding from the side walls of casing 6.The fingertip cushion supporting plate 10 has at its forward end, afingertip stopper 12 which is lower than the height of the nail of afingertip 1 so as not to catch the long nail. The fingertip cushionsupporting plate 10 also has guide walls 14 at the two sides thereofwhich are separated from each other by a distance slightly larger thanthe standard width of the fingertip 1. A fingertip cushion 1a is placedon the central portion of a fingertip cushion supporting surface 11. Thecentral portion of the fingertip cushion supporting surface is providedwith a through-hole 14. A light-emitting element 15 is mounted on thefingertip cushion supporting surface 11 below the through-hole 14.

A nail supporting plate 20 which acts as a nail supporting member, isprovided in the upper portion of the casing 6. The nail supporting plate20 has a nail supporting surface 21 which is inclined at substantiallythe same angle as that at which the fingertip cushion supporting surface11 is inclined in a normal state. A hole 22 is formed in the nailsupporting plate 20 at a position at which it faces the hole 14. Alight-receiving element 26 is mounted on the nail supporting plate 20behind the hole 22.

A constant-load spiral spring 30 is accommodated in the casing 6 belowthe fingertip cushion supporting plate 10. The inner end of the spiralspring 30 is pressed onto a rotary shaft 32 rotatably supported by arms31 handing from the fingertip cushion supporting plate by virtue of itselasticity, and an outer end 30a thereof is fixed to the front surfaceof the casing 6.

When the measurement is to be conducted, the fingertip 1 is advanceduntil a forward end 1b thereof abuts against the stopper 12, as shown inFIG. 1. At that time, the fingertip cushion supporting plate 1 islowered by a distance corresponding to the thickness of the fingertip 1,and the constant-load spiral spring 30 is thereby unwound by a lengthcorresponding to the lowering stroke of the spring 30. Further, theconstant load characteristics of the spring enable the pressuregenerated by the winding force of the spring, to be maintained at anappropriate fixed value. In this way, the fingertip 1 is applied with anappropriate pressure sufficient to keep it in place and yet not togenerate distortion in the pulse waveform, which is caused by theexcessive pressure applied to the fingertip due to variations in thethickness of the fingertips resting on the fingertip cushion supportingplate. The light emitted from the light-emitting element 15 passesthrough the hole 14, is transmitted through the fingertip 1, and isreceived by the light-receiving element 26 which is disposed in opposedrelationship with the light-emitting element 15. In the light-receivingelement 26, the received light is converted into electrical signals. Thethus-generated stable signals accurately representing the changes in thevolume of blood in a vein, are supplied to the body (not shown) of afingertip pulse wave sensor.

FIGS. 3 and 4 show a second embodiment of the present invention. In thisembodiment, a fingertip cushion supporting plate 40 is provided in acasing 9. The fingertip cushion supporting plate 40 has a guide surface44, guide walls 43 and a fingertip stopper 42. The central portion ofthe fingertip cushion supporting surface 41 is provided with a hole 47.A light-emitting element 46 is disposed on the fingertip cushionsupporting surface 41 behind the hole 47.

A nail supporting plate 50 is disposed in the casing 9 substantiallyparallel to the fingertip cushion supporting plate 40. Bearing portions51 provided on the two sides of the nail supporting plate 50, are hingedon a pin 52 extending between the side walls of the casing 9. A coilspring 53 is provided on the pin 52. The one end of the coil spring 53is fixed to the upper surface of the casing 9, and the other end thereofis fixed to the nail supporting plate 50. The coil spring 53 urges thenail supporting plate 50 in the direction in which a curved rear endportion 50a thereof abuts against the upper surface of the casing 9.

The nail supporting plate 50 has a nail supporting surface 54. A hole 55is formed in the nail supporting plate 50 at a position at which itfaces the hole 47. A light-receiving element 56 is mounted on the nailsupporting plate 50 behind the hole 55.

When the fingertip 1 is inserted along the guide surface 44 and is thenadvanced until it abuts against the fingertip stopper 42, the nailsupporting plate 50 is raised against the pressure of the coil spring 53by a distance corresponding to the thickness of the fingertip inserted.At this time, since the rear portion of the nail supporting plate 50 ishinged, the amount of pivot of the coil spring 53 is small with respectto the stroke of the nail supporting plate 54 by virtue of theprinciples of lever. Further, constant load characteristics are ensuredby using a sufficient number of turns (e.g., in the coil spring), andthe pressure applied by the spring can be maintained at a constant valuewith respect to the variations in the thickness of the fingertips.

In any of the above-described embodiments, the fingertip cushionsupporting member and the nail supporting member, have a plate-likeform. However, the fixed member may also be shaped in a block-like form.Air springs and other types of mechanical springs may be employed as theelastic body which is adapted to maintain the pressure at a constantvalue with respect to the variations in the pivot stroke which arecaused by the differences in individuals. Furthermore, the firstembodiment may also employ a coil spring, and the second embodiment mayalso use a constant-load spiral spring.

What is claimed is:
 1. A fingertip pulse wave sensor, comprising:acasing; a fingertip cushion supporting member fixed relative to saidcasing and having a fingertip cushion supporting surface; a nailsupporting member having a forward portion and a rearward portionadjacent to the forward portion, the nail supporting member being hingedto said casing and having a nail supporting surface which is disposed inopposed relationship with said fingertip cushion supporting surface at adistance smaller than the standard thickness of fingertips; an elasticmeans for urging said hinged nail supporting member toward saidfingertip cushion supporting surface in such a manner that the pressureapplied by said elastic means becomes substantially constant withrespect to the variations in the pivot stroke caused by the variationsin the thickness of said fingertips resting on said fingertip cushionsupporting member; a light-emitting element disposed on one of said nailsupporting surface at the forward portion thereof and said fingertipcushion supporting surface; and a light-receiving element disposed onthe other of said nail supporting surface at the forward portion thereofand said fingertip cushion supporting surface; the nail supportingsurface being hinged to the casing at a point situated rearwardly of oneof the light-emitting element and the light-receiving element disposedthereon.
 2. A fingertip pulse wave sensor according to claim 1, whereinsaid elastic means comprises a coil spring which is mounted on a hingedrear portion of said nail supporting member.
 3. A fingertip pulse wavesensor, comprising:a casing; a fingertip cushion supporting memberhaving a forward portion and a rearward portion adjacent to the forwardportion, the fingertip cushion supporting member being hinged to saidcasing and having a fingertip cushion supporting surface; a nailsupporting member fixed relative to said casing and having a nailsupporting surface which is disposed in opposed relationship with saidfingertip cushion supporting surface at a distance smaller than thestandard thickness of fingertips; an elastic means for urging saidhinged fingertip cushion supporting member toward said nail supportingsurface in such a manner that the pressure applied by said elastic meansbecomes substantially constant with respect to the variations in thepivot stroke caused by the variations in the thickness of saidfingertips resting on said fingertip cushion supporting member; alight-emitting element disposed on one of said fingertip cushionsupporting surface at the forward portion thereof and said nailsupporting surface; and a light-receiving element disposed on the otherof said fingertip cushion supporting surface at the forward portionthereof and said nail supporting surface; the fingertip cushionsupporting surface being hinged to the casing at a point situatedrearwardly of one of the light-emitting element and the light-receivingelement disposed thereon.
 4. A fingertip pulse wave sensor according toclaim 3, wherein said elastic means comprises a coil spring which ismounted on a hinged rear portion of said fingertip cushion supportingmember.